For convenience of description the present invention will be described with reference to a high speed paging system which has been evolved from, and is compatible with, POCSAG or CCIR Radiopaging Code No 1, the details of which are disclosed in "The Book of the CCIR Radiopaging Code No 1" available from the Secretary, RCSG, British Telecom, Radiopaging, 23 Howland Street, London W1P 6HQ. In accordance with POCSAG, paging messages are transmitted in batches each of which comprises a synchronisation code word and 8 frames each comprising 2 code words. Each pager or secondary station is assigned to a particular frame which means that if a paging signal is transmitted by a primary station for a particular pager, it will be in a predetermined one of the 8 frames. Each POCSAG pager is controlled to power up to detect the synchronisation (sync) code word and again for the duration of its particular frame in order to be able to receive paging messages which are or commence with an address code word. In the case of messages, the message code words are concatenated with the address code word. The POCSAG address and message code words each comprise 32 bits of which the first bit has a value "0" for an address code word and "1" for a message code word. Both types of code words at bit positions 22 to 31 comprise cyclic redundancy check (CRC) bits and the bit position 32 provides an even parity bit. At the end of a message any waiting address code words will be transmitted, starting with the first appropriate to the first free frame. In the absence of an appropriate address code word, an idle code word, which is a non-allocated address code word is transmitted.
The high speed paging system which has evolved from POCSAG has a format comprising batches constructed with code words in frames following a synchronisation code word. In the high speed system operating at, say, 6400 bits per second, the number of code words in a batch is increased compared to the number, 17, in a batch when operating at 1200 bits per second. In order to ensure compatibility a fixed cycle period of 6.8 seconds is maintained for all formats. Depending on whether messages are numeric or alpha-numeric only will depend on the number of batches, for example 3 or 15, there are to a cycle.
Additionally the high speed paging system requires the synchronisation code word to be a 32 bit, 1200 bit per second code word conforming to the same structure as that used for POCSAG. However, unlike normal POCSAG the synchronisation code word defines for the pager or secondary station the information type, frame format, code word structure and bit rate in its batch. Whilst the use of different synchronisation code words will enable a pager to adapt its receiver and decoder to receive and decode different signal formats, it is desired for a pager network to be able to offer various services and features to the pager user, if necessary by offering different services and features to the different pager users.